1. Field of the Invention
The present invention relates to a photoreceptor and a manufacturing method thereof, a method of image formation, image forming apparatus, and a process cartridge.
2. Discussion of the Background
Recently, organic photoconductors (photoreceptors) have been used in place of inorganic photoreceptors for a photocopier, a facsimile machine, a laser printer and a multi-functional device thereof in light of performances and advantages. Specific reasons for this replacement are, for example, (1) good optical characteristics, for example, width of the range of optical absorption wavelength and size of the amount of absorption of light; (2) electric characteristics, for example, high sensitivity and stable chargeability; (3) a wide selection of materials; (4) ease of manufacturing; (5) inexpensive cost; and (6) toxic-free property.
In addition, the trend of the size reduction of an image forming apparatus has accelerated the size reduction of an image bearing member (photoreceptor). Therefore, with the advancement of high speed performance and maintenance-free, a photoreceptor (image bearing member) having a high durability has been desired. From this point of view, an organic photoconductor is soft in general and easy to wear down because the surface layer thereof is mainly made of a low molecular weight charge transport material and an inert polymer. Thus, an organic photoconductor repetitively used in the electrophotography process tends to be abraded under mechanical stress by a developing system or a cleaning system. Currently, this is a significant problem because the cleaning property of the photoreceptor should be improved to deal with size-reduced toner particles demanded for production of quality images so that usage of hard rubber for the cleaning blade and an increase in the contact pressure between the cleaning blade and the photoreceptor are inevitable.
Such abrasion of a photoreceptor causes deterioration of electric characteristics, for example, the sensitivity and the chargeability, resulting in production of abnormal images having, for example, low image density and background fouling. Local abrasion damage to a photoreceptor causes streaks on an image due to bad cleaning performance on the photoreceptor. Therefore, such abrasion and damage determine the working life of a photoreceptor and lead to replacement thereof.
Therefore, the reduction of the abrasion amount is inevitable to manufacture an organic photoconductor having a long working life and creates the most imminent issue in this field.
With regard to improvement on the anti-abrasion property of the photosensitive layer, for example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637 describes a technology of using a curable binder resin in the surface layer. JOP S64-1728 describes a technology of using a charge transport polymer. JOP H04-281461 describes a technology of dispersing an inorganic filler in the surface layer. Among these technologies, the curable binder resin in the surface layer has poor compatibility with a charge transport material and impurities such as a polymerization initiator and non-reacted residual group tend to raise the residual voltage, which leads to deterioration of image density.
In addition, the technology of using the charge transport polymer and the technology of dispersing an inorganic filler in the surface layer improve the abrasion resistance and improvement on these technologies are demanded for further extended period of use of an organic photoconductor.
Furthermore, the technology of dispersing an inorganic filler in the surface layer tends to cause trap of charge carrier existing on the surface of the inorganic filler, which leads to a rise of residual voltage. Therefore, image density easily decreases. Thus, a measurement to deal with such image density reduction is required. In conclusion, these technologies need improvement on comprehensive durability suitable for an organic photoconductor including electric durability and mechanical strength.
Furthermore, Japanese patent No. (hereinafter referred to as JP) 3262488 describes a technology of improving abrasion resistance and damage resistance by using a photoreceptor including multi-functional cured acrylate monomer. JP 3262488 has a description about usage of the multi-functional cured acrylate monomer in a protection layer provided on the photosensitive layer but inclusion of a charge transport material in the protection layer is just mentioned with no specific description. Furthermore, the multi-functional cured acrylate monomer has a compatibility problem with a charge transport material having a low molecular weight simply contained in the surface layer, thereby causing precipitation of the low molecular weight charge transport material and white turbidity. This easily degrades the mechanical strength.
Furthermore, to be specific, the photoreceptor is formed by reaction of monomers in the state in which binder polymers are contained. This creates problems of insufficient curing reaction, and compatibility between the cured material and the binder resin. Thus, a rough surface is easily manufactured due to the phase separation in the curing reaction, which leads to bad cleaning performance.
Abrasion resistance technologies instead of the technologies described above have been thus introduced. For example, JP 3194392 describes a technology of forming a charge transport layer by a liquid application containing a monomer having a carbon-carbon double bonding, a charge transport material having a carbon-carbon double bonding and a binder resin. This binder resin includes a resin having a carbon-carbon double bonding with reactivity with the charge transport material and another resin having no carbon-carbon double bonding with no reactivity with the charge transport material. This photoreceptor has a good combination of abrasion resistance and electric characteristics. However, the resin having no carbon-carbon double bonding with no reactivity with the charge transport material has poor compatibility with a cured material produced in the reaction of the monomer and the charge transport material. Thus, a rough surface is easily formed while in the phase separation and cross-linking, which results in bad cleaning performance.
In addition to the blocking of monomer curing by, the binder resin as described above, the monomers specified in the technology are two functional, meaning that the number of functional groups is insufficient to obtain a sufficient cross linking density. Therefore, the obtained photoreceptor does not have sufficient abrasion resistance.
Furthermore, when the binder resin having a reactivity is used, it is difficult to have a good combination of the bonding amount of the charge transport material and the cross-linking density since the monomer and the binder resin have a small number of functional groups. Therefore, the obtained photoreceptor has insufficient electric characteristics and abrasion resistance.
A surface layer did not used to be formed by simple application of a liquid application of monomer and cross-linking and/or elongation reaction without using a solid resin from which the status of the surface layer is relatively easy to predict. However, recently a technology of improving both the electric characteristics and the abrasion resistance has been introduced in which the surface of a cured cross-linked resin layer is formed by curing a radical polymerizable monomer (1) having at least 3 functional groups with no charge transport structure and a radical polymerizable compound (2) having a charge transport structure (refer to, for example, JOPs 2004-302450, 2004-302451, and 2004-302452).
The radical polymerizable monomer (1) is easily available from the market but the purity thereof is generally low and contains impurities deriving from catalysts, etc. These degrade abrasion resistance and electric characteristics.
In addition, some of the among radical polymerizable monomers (1) are not safe. This safety problem is ascribable to the impurities contained in the radical polymerizable monomer (1), particularly residuals relating to the catalyst.